In a VoIP system, devices may communicate through the exchange of call control signaling. VoIP signaling protocols (e.g. SIP, MGCP, and Megaco/H.248) may define a command message transmission timeout. In an example embodiment, this timeout may be defined when the protocol is used over an unreliable transport, for example User Datagram Protocol (UDP). A command message transmission timeout may occur when a final response message is not received within a timeout period. A command message may be retransmitted a number of times with an exponential backoff between retransmissions. A transaction timeout may occur when all retransmissions of a command message have been exhausted without receiving a final response message.
There may be two different classes of VoIP systems, including Decomposed Gateways, as supported by MGCP, NCS, TGCP, and Megaco/H.248, and Intelligent Endpoints, as supported by SIP and H.323.
In the decomposed gateway architecture, there may be a Media Gateway Controller (MGC) and a Media Gateway (MG). In the Intelligent Endpoint architecture, there may be endpoints (e.g. SIP User Agents), and intermediaries (e.g. SIP proxies). Both types of systems may support client-server style signaling where a client may send a transaction request to a server and expect to receive zero, one or more provisional response messages followed by a final response message for the transaction.